1. Field of the Invention
The present invention relates generally to methods for the non-destructive evaluation of the condition of pipes, tubes, cylindrical shells and the like. More specifically, the present invention relates to a method for determining the mean radius of a pipe or tube using guided mechanical waves from a single location on the tubular wall structure.
2. Description of the Related Art
Many industrial structures, frames, conduits, flow columns and the like, are constructed in cylindrical configurations that are often difficult to access. Nonetheless, there is a strong interest in the industry to carry out inspections of such pipe and tube structures for the purpose of determining their remaining life or the nature of any maintenance requirements. Frequently these inspection methods involve the use of non-destructive evaluation (NDE) techniques that are applied to the inside diameter (ID) of the tube or pipe to determine the remaining wall thickness for the structure. Traditional inspection methods generally require an ID probe that is pulled or pushed through the pipe or tube in a progressive manner. Thickness data is acquired from a sequence of incremental positions within the tube or pipe.
There are, however, many environments where an entire length of pipe is not easily accessible from the inside diameter. Some NDE methods are capable of operating on the outer wall of such pipes and tubes but frequently such outer walls are even less accessible than the inside diameter. Heat exchangers are good examples of structural environments where the entire length of a pipe or tube that requires inspection is not accessible. These structures typically involve tube sheets that incorporate sometimes hundreds of U-shaped lengths of tubing. This makes both the interior and exterior walls of the tubing difficult to access for inspection purposes. In many instances the only access points are the terminals of the tubing that are presented at the surface of the tube sheet. It would be difficult to carry out any type of progressive inspection technique in such a structural environment.
In addition to problems with accessibility, progressive inspection techniques often result in large quantities of data that must first be analyzed to determine discrete wall thickness values for a range of locations in the pipe wall. While in some instances this discrete information might be relevant, it is more often the case that an average wall thickness is the desired quantity for determining the remaining service life of a pipe or tube system. Taking a range of values for discrete locations within a pipe or tube and then averaging those values to obtain a quantity for the entire pipe or tube becomes a burdensome task when it is only an average value that is desired from the start.
When the inside diameter of a pipe or tube is accessible, ultrasonic waves and/or magnetostrictively induced mechanical waves can be used to inspect the pipe or tube wall for a determination of the average wall thickness. The types of waves most suitable for an inspection down the length of a target tube are those that propagate longitudinally through the walls using the walls as a wave guide. If these waves could be generated from a single location and could be analyzed from the same or a nearby location, then the goal of acquiring information on the average wall thickness could be more easily achieved.
In general, it would be desirable to carry out inspections of various industrial pipes and tubes from a single location on the inside diameter of the structure. It would be desirable to achieve such an inspection with the goal of obtaining an average thickness or mean radius value that permits an inference of the remaining life of the tube or pipe structure. It would be desirable to achieve such an inspection using familiar inspection techniques such as ultrasonic and magnetostrictive interrogation probes. It would be desirable if a single analysis of wave propagation within a length of pipe could be used to determine the average wall thickness for that length of pipe.
Even with existing techniques for the inspection and determination of wall thicknesses in pipes and tubes, it is often desirable to have some baseline information about the original geometries of the pipe or tube. In this manner, deviations from a baseline value for wall thickness at each discrete test point can be used to calculate the wall thinning that may have occurred at that point. In a system that attempts to measure average wall thickness from a single point on the inside diameter of a pipe or tube, it would also be desirable to have baseline data to facilitate a comparison and the calculation of a new wall thickness average.